Ingress Noise Inhibiting Network Interface Device and Method for Cable Television Networks

ABSTRACT

Ingress noise from subscriber equipment is mitigated or prevented from reaching a cable television (CATV) network. All upstream signals including ingress noise are initially transmitted to the CATV network whenever their instantaneous power exceeds a threshold which typically distinguishes ingress noise from a valid upstream signal. Whenever the instantaneous power is below the threshold, ingress noise is blocked from reaching the CATV network. A gas tube surge protection device is included to resist component destruction and malfunction arising from lightning strikes and other high voltage, high current surges.

This invention relates to cable television (CATV) networks, and more particularly to a new and improved CATV network interface device which interconnects subscriber equipment at a subscriber's premises to the CATV network infrastructure. The present network interface device offers an improved capability for inhibiting the amount of undesirable ingress noise introduced from subscriber equipment to the CATV network without diminishing the information content of valid upstream signals and while achieving use compatibility with most CATV networks without regard to upstream communication protocols or unique equipment used in the CATV network.

BACKGROUND OF THE INVENTION

CATV networks supply high frequency “downstream” signals from a main signal distribution facility, known as a “headend,” through the CATV network infrastructure to the homes and offices of subscribers to the CATV signal distribution services. The infrastructure of a typical CATV network includes interconnected coaxial cables, signal splitters and combiners, repeating amplifiers, filters, trunk lines, cable taps, drop lines and other signal-conducting devices. The downstream signals are supplied to the subscriber equipment, such as television sets, telephone sets and computers, to cause them to operate.

In addition, most CATV networks also transmit “upstream” signals from the subscriber equipment back to the headend of the CATV network. For example, a set top box allows the subscriber to select programs for display on the television set. Upstream signals are sent from the set top box to the headend signal-delivering equipment that then transmits the selected downstream signal to the subscriber. As another example, two-way communication is essential when using a personal computer connected through the CATV infrastructure to the public Internet. As a further example, voice over Internet protocol (VOIP) telephone sets use the CATV infrastructure and the public Internet as the medium for transmitting two-way telephone conversations. Such two-way signal transmission (upstream and downstream) is therefore an essential requirement for modern CATV networks.

To be effective, a CATV network must use filters and other components which reduce or eliminate unwanted signals that enter the CATV network from external sources. These undesirable external signals, known as “ingress noise,” have the effect of degrading valid signals, if measures are not taken to suppress or otherwise limit the amount of ingress noise in a CATV network.

The most intense frequency of undesirable ingress noise signals is in the frequency band of 0-15 megahertz (MHz). Valid upstream signals are within the frequency band of 5-42 MHz, which overlaps with the frequency band of the most intense ingress noise. It is therefore impossible or extremely difficult to filter undesirable ingress noise from valid upstream signals when the two electrical signals occupy the same frequency band and both signals may originate at approximately the same location at the subscriber premises. Valid downstream signals are within the frequency band of 54-1000 MHz, so the ingress noise, typically in the 0-15 MHz frequency band, is usually suppressed by filters in the downstream frequency band.

Even though the ingress noise is typically in a frequency band different from the downstream frequency band, ingress noise can still have adverse influence on both valid downstream and upstream signals. Ingress noise from individual subscribers tends to funnel together and accumulate as a substantial underlying level of base noise on the CATV network. Valid signals must be distinguished from this base level noise, usually by amplifying the valid signals above the base noise level. A high level of base noise may cause signal amplifiers to clip or distort both the valid downstream and upstream signals during amplification and retransmission of those signals, thereby reducing the information contained in those valid signals. A reduction in the information contained in the signals diminishes the quality of service experienced by the subscriber and may even inhibit the delivery of services to subscribers.

There are many potential sources of ingress noise in the environment of a typical CATV network. However, the typical CATV network has a relatively high immunity to ingress noise because the CATV network infrastructure is essentially constructed by professionals using high quality equipment and techniques. However, the situation is usually considerably different at the subscriber premises. The quality of the subscriber equipment, the type and integrity of the signal conductors within the consumer premises, the effectiveness and quality of the connections between the subscriber equipment and the signal conductors, and the presence of many other types of electrical devices which emit noise, such as electric motors, radios and consumer appliances, become sources of ingress noise at the subscriber premises over which the CATV service provider has no control.

Even though the CATV service provider may have little control over the sources of ingress noise at the subscriber premises, the CATV service provider is nevertheless responsible for the quality of service, at least from the perspective of subscribers. Therefore, different types of ingress noise inhibiting devices have been devised for use with CATV networks to attempt to suppress ingress noise entering the CATV network from the subscriber premises.

One type of known ingress noise inhibiting device relies on downstream signals generated at the headend in accordance with the communication protocol to close an electronic switch at predetermined times and under predetermined circumstances to establish an upstream communication path for valid upstream signals. Once the upstream communication is established, the subscriber equipment is permitted to transmit upstream signals in synchronization with the establishment of the path. The upstream signals from subscriber equipment can only be communicated at those times established by the communication protocol. At all other times, all upstream signals, including ingress noise, are blocked and prevented from entering the CATV network. The times when the electronic switch is closed are established by the communication protocol, and those time periods may not correspond with the times when the subscriber makes programming selections, desires to transmit upstream signals, or is talking during a telephone conversation, for example.

Protocol-responsive ingress noise inhibiting devices have the potential to delay the transmission of the upstream communications, and as a result, the response thereto, because the upstream communications path is only established during those predetermined times set by the communication protocol. The times set by the communication protocol do not usually correspond with the times when the user wishes to transmit valid upstream signals. The resulting delays are perceived by the subscriber as deficient responsiveness and a reduction in the quality of service. Furthermore, since the time intervals for transmitting upstream signals is preestablished by the communication protocol, the closed electronic switch permits ingress noise to enter the CATV network during those times when there are no subscriber upstream signals to transmit, thereby allowing ingress noise to enter the CATV network.

A further difficulty with such protocol-responsive ingress noise inhibiting devices is that they are specifically useful only in those types of CATV networks which require a specific communication protocol. Because not all CATV networks operate on the same basis, protocol-controlled ingress noise inhibiting devices do not have wide applicability to a variety of different types of CATV networks and CATV service providers. In addition, synchronizing the subscriber equipment to the CATV network protocol requires specialized equipment.

A related type of ingress noise inhibiting device permits upstream communications in only one or more narrow band pass frequencies, for example at 11 and/or 26 MHz. Filters are employed to block any ingress noise within the other ranges of the 5-42 MHz upstream frequency band and the 0-15 MHz typical ingress noise frequency band. Although such bandpass ingress noise inhibiting devices are effective in suppressing the ingress noise outside of the bandpass frequencies, ingress noise is still able to enter the CATV network at the selected bandpass upstream frequencies. Further, the use of such narrow frequency bandpass ingress noise inhibiting devices is applicable only to those types of CATV networks which limit the frequency of valid upstream signals to preselected frequency bands. The use of preselected upstream frequency bands for valid upstream signals is not universally applicable to a variety of different types of CATV networks and CATV service providers.

Another type of ingress noise inhibiting device is one which responds to an auxiliary out-of-band signal to close an electronic switch and establish an upstream communication path. For example, the auxiliary out-of-band signal may be a 1 MHz tone, which falls outside of the upstream frequency band. The subscriber equipment generates this out-of-band signal whenever it wishes to transmit an upstream communication. The ingress noise inhibiting device responds to the out-of-band signal and closes the electronic switch to establish the communication path for the upstream signal in the 5-42 MHz frequency band. Typically, the out-of-band signal remains present while the upstream signal is transmitted. When the out-of-band signal is not generated, the electronic switch opens to block the communication path, thereby preventing ingress noise from entering the CATV network. Such ingress noise inhibiting devices require the subscriber equipment and set-top boxes to have the additional functionality of generating, recognizing and responding to the out-of-band signal. Such equipment is not common, and adds to the cost and difficulty of the equipment support operations of the CATV service provider. Furthermore, the ingress noise inhibiting device also requires additional components to function in a frequency band different from the normal 5-42 MHz upstream frequency band in which other components operate. Lastly, ingress noise in the out-of-band frequency range can also cause the electronic switch to close and establish the upstream communication path when there is no valid upstream signal to transmit, thereby admitting ingress noise on to the CATV network.

Other types of ingress noise inhibiting devices attempt to distinguish ingress noise from valid upstream signals, on the basis of characteristic differences in the ingress noise signals and the valid upstream signals. Ingress noise is characterized by erratic amplitude and timing variations, while valid upstream signals are characterized by regular amplitude and consistent timing characteristics. Valid upstream signals are frequently transmitted in the form of packets, which are defined by the presence and absence of high-frequency pulses that constitute bits of a digital signal. The typical packet includes a preamble with a series of high-frequency pulses representing digital bits which define the start of the packet. Certain packet-responsive ingress noise inhibiting devices attempt to recognize the preamble, and in response, close an electronic switch to establish a pathway for the valid upstream signal. Distinguishing the preamble requires time to recognize its regular timing and amplitude characteristics. The amount of time available to perform such recognition may not always be adequate, particularly when the high-frequency pulses of the preamble are of low or moderate strength. Under those circumstances, the upstream communication path may not be established quickly enough to transmit the body of substantive information carried by the packet, thereby resulting in loss of some of the information and the perception of a diminished quality of service. Not all CATV networks operate on a digital packet communication protocol, so the applicability of packet-responsive ingress noise inhibiting devices is not universal.

Another difficulty arising from some known ingress noise inhibiting devices involves attempting to switch filters in and out of electrical connection to establish the upstream communication path and to suppress the ingress noise when the upstream communication path is not established. Switching filters in and out of circuit connection requires a finite amount of time for the energy storage inductors and capacitors of such filters to store the necessary energy and to achieve stabilized operability to perform filtering. Of course, the time required to store the energy, achieve stability and commence filtering the signals may also result in truncating or diminishing the information content of the upstream signals.

Still another type of ingress noise inhibiting device attempts to distinguish between spurious ingress noise and valid upstream signals on the basis of their energy content. Such devices function by integrating the power of the signals over time to arrive at an energy value. The assumption is that the power of valid upstream signals, when integrated, will represent an energy content sufficiently greater than the integrated power or energy of spurious ingress noise signals, because valid upstream signals have sustained energy while spurious noise signals have erratic low energy. The sustained length of valid upstream signals integrates to recognizable energy level, while the short and erratic length of ingress noise integrates to a much lesser energy level. After the time period required for integrating the power into energy, the energy level is compared to a predetermined threshold energy level which has been selected to represent a valid upstream signal. If the energy level exceeds the predetermined threshold energy level, an electronic switch is closed to establish the upstream communication path. If the integration of the power results in an energy level which is less than the predetermined threshold energy level, it is assumed that the signal is ingress noise, and the electronic switch remains open to prevent any signals from reaching the CATV network.

To integrate the power level of upstream signals into energy, a time delay is required before valid upstream signals can be transmitted to the CATV network. This delay in transmitting valid upstream signals presents the possibility that some of the valid upstream signal will be lost or truncated before the upstream communication path is established.

SUMMARY OF THE INVENTION

The CATV network interface device and method of this invention are effective in mitigating ingress noise over the entire 5-42 MHz upstream frequency band of a CATV network, and do so while transmitting valid upstream signals almost instantaneously to avoid loss of information content. The valid upstream signals are transmitted without requiring a time delay sufficient to determine energy content. Consequently, valid upstream signals are transmitted almost immediately to the CATV network after the subscriber equipment generates those signals. The almost instantaneous transmission of valid upstream signals avoids the risk of loss of information content. The present device and method is not limited in its applicability or use to any particular type of CATV network or any particular type of communication protocol used on a CATV network. The present ingress noise inhibiting network interface device and method do not require synchronization with CATV network communication protocol or require special functionality in subscriber equipment to synchronize valid upstream signals with the CATV network communication protocol. No out-of-band signaling or functionality is required to implement the present invention. Upon termination of the valid upstream signal, the present device and method quickly revert to a condition which effectively blocks ingress noise from the CATV network. No filters are switched into or out of electrical connection. When blocking ingress noise from the CATV network, the connections to the CATV network and to the subscriber equipment are terminated into characteristic impedances to minimize reflected signals that detract from valid signals.

In accordance with these and other features, one aspect of the present invention involves a network interface device which has an upstream noise mitigation circuit that mitigates the ingress of noise from subscriber equipment into a cable television (CATV) network. The CATV network transmits downstream signals in a first frequency band from a headend to the subscriber equipment and transmits upstream signals in a second different frequency band from the subscriber equipment to the headend. The ingress noise mitigation circuit comprises a downstream filter which filters downstream signals before delivery to the subscriber equipment, and an upstream filter which filters upstream signals before delivery to the CATV network. A detector determines an instantaneous level of power of the upstream signals. A threshold circuit establishes a predetermined threshold power level which distinguishes typical ingress noise from valid upstream signals. A comparator compares the instantaneous power level of the upstream signal with the threshold power level and asserts a trigger signal when the instantaneous power level exceeds the threshold power level. A switch is connected to the upstream filter and terminates the upstream filter in a characteristic impedance to block upstream signals and to prevent or minimize signal reflection when in a normal position. The switch conducts the upstream signals from the subscriber equipment to the CATV network when in an activated position. The switch assumes the activated position when the instantaneous power level exceeds the threshold power level, as represented by the assertion of the trigger signal, and assumes the normal position under usual circumstances and when the instantaneous power level is less than the threshold power level, represented by the de-assertion of the trigger signal. By activating the switch immediately after the instantaneous power content of the upstream signal exceeds the threshold power level, there is little possibility or opportunity for the information contained in the upstream signals to be lost, truncated or diminished.

Other aspects of the network interface device involve a timer which is operative to maintain the switch activated for a predetermined time period after the instantaneous power level exceeds the threshold power level and which is operative to return the switch to the normal position after expiration of a predetermined time period after the switch is activated. The predetermined time is sufficient to transmit a single valid maximum-length upstream signal. The continued presence of energy from multiple valid sequential upstream signals maintains the switch in the activated position to permit transmission of those signals. Should ingress noise have an instantaneous power level sufficient to exceed the threshold power level, the switch will quickly resume the normal position and prevent further transmission of the ingress noise after the ingress noise dissipates.

Additional aspects of the network interface device involve first and second upstream filters which filter the upstream signals before delivery to the CATV network, and first and second switches connected to the first and second upstream filters. The first and second switches assume activated positions in response to the instantaneous power content exceeding the threshold power level and assume normal positions in response to the instantaneous power content remaining below the threshold power level. In the normal positions, the two switches terminate the filters through characteristic impedances to preventingress noise from the subscriber equipment from reaching the CATV network. In the activated positions, the two switches conduct the upstream signals through the first and second upstream filters.

Another aspect of the present invention is a network interface device which includes a gas tube surge protection device. The gas tube surge protection device shunts high voltage and high current surges, such as those arising from lightning, from CATV network components and the subscriber equipment.

A method of mitigating upstream noise originating from subscriber equipment is a further aspect of the present invention. The method involves filtering upstream signals including upstream noise to confine the frequency of the upstream signals to an upstream frequency band, determining an instantaneous power content of the upstream signals, establishing a threshold power level which typically distinguishes ingress noise from valid upstream signals, comparing the instantaneous power content of the upstream signals to the threshold power level, blocking the filtered upstream signals from the CATV network when the instantaneous power content is less than the threshold power level, and conducting the filtered upstream signals to the CATV network when the instantaneous power content is at least equal to the threshold power level.

Other features of the method involve conducting upstream signals to the CATV network for a predetermined time period after the instantaneous power content exceeds the threshold power level. The instantaneous power content is integrated over a predetermined integration time to arrive at an integration value. If the integration value is less than a predetermined threshold energy level, thereby signifying ingress noise, the upstream communication path is blocked to prevent the ingress noise from reaching CATV network after the predetermined integration time has elapsed. If the integration value is greater than the predetermined threshold energy level, thereby signifying the presence of a valid upstream signal, the upstream communication path is maintained for the time duration of a single valid maximum-length upstream signal. If the integration value is greater than the predetermined threshold energy level, thereby signifying the presence of a valid upstream signal, and the instantaneous power of the valid upstream signal continues after the time duration of a maximum-length upstream signal, the valid upstream signal is constituted by a sequence of multiple valid upstream signals. In this circumstance, the upstream communication path is maintained for the time duration of the multiple valid upstream signals. In the respective cases of a single valid upstream signal or multiple sequential valid upstream signals, maintaining the upstream communication path for the duration of a single maximum-length upstream signal assures or for the duration of the multiple sequential valid upstream signals assures that the information contained in the valid upstream signals will be fully and accurately transmitted without truncation or other loss of information. After the time duration of a single valid maximum-length upstream signal or the time duration of a sequence of multiple valid upstream signals, the upstream communication path is terminated to block ingress noise from entering the CATV network.

Other features and aspects of the invention, and a more complete appreciation of the present invention, as well as the manner in which the present invention achieves the above and other improvements, can be obtained by reference to the following detailed description of presently preferred embodiments taken in connection with the accompanying drawings, which are briefly summarized below, and by reference to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a network interface device which incorporates the present invention and a block diagram of subscriber equipment shown connected to a CATV network through the network interface device located at a subscriber's premises.

FIG. 2 is a block diagram of portions of a typical CATV network, with multiple network interface devices of the type shown in FIG. 1 connected by drop cables to cable taps, as well as other aspects of the CATV network.

FIG. 3 is a block diagram of basic functional components within the network interface device shown in FIG. 1.

FIGS. 4, 5 and 6 contain multiple waveform diagrams on a common time axis, illustrating the functional features of an upstream noise mitigation circuit of the network interface device shown in FIG. 3.

FIG. 7 is a block diagram of basic functional components of an upstream noise mitigation circuit which is an alternative to that shown in FIG. 3.

FIGS. 8, 9 and 10 contain multiple waveform diagrams on a common time axis, illustrating the functional features of the upstream noise mitigation circuit shown in FIG. 7.

DETAILED DESCRIPTION

A network interface device 10 which incorporates the present invention is shown in FIG. 1. The network interface device 10 includes a housing 12 which encloses internal electronic circuit components (shown in FIGS. 3 and 7). A mounting flange 14 surrounds the housing 12, and holes 16 in the flange 14 allow attachment of the interface device 10 to a support structure at a subscriber's premises 18.

The interface device 10 is connected to a conventional CATV network 20, which is shown in a typical topology in FIG. 2. Downstream signals 22 originate from programming sources at a headend 24 of the CATV network 20, and are conducted to the interface device 10 in a sequential path through a main trunk cable 26, a signal splitter/combiner 28, secondary trunk cables 30, another signal splitter/combiner 32, distribution cable branches 34, cable taps 36, and drop cables 38. Upstream signals 40 are delivered from the network interface device 10 to the CATV network 20, and are conducted to the headend 24 in a reverse sequential path. Interspersed at appropriate locations within the topology of the CATV network 20 are conventional repeater amplifiers 42, which amplify both the downstream signals 22 and the upstream signals 40. Conventional repeater amplifiers may also be included in the cable taps 36. The cable taps 36 and signal splitter/combiners 28 and 32 divide a single input downstream signal into separate downstream signals, and combine multiple upstream signals into a single upstream signal.

The network interface device 10 receives the downstream signals 22 from the CATV network 20 at a network connection port 44. The downstream signals 22 are either passive or active. Passive downstream signals are those signals which are conducted through the interface device 10 without amplification, enhancement, modification or other substantial conditioning. The passive downstream signals are delivered from a passive port 45 to passive subscriber equipment, such as a voice modem 46 connected to a telephone set 48, or an embedded multimedia network interface device (EMTA, not shown), located at the subscriber premises 18. Active downstream signals are those signals which are amplified, filtered, modified, enhanced or otherwise conditioned by power-consuming active electronic circuit components within the interface device 10. The conditioned active downstream signals are divided into multiple copies and delivered from a plurality of active ports 50, 52, 54 and 56 to active subscriber equipment located at the subscriber premises 18, such as television (TV) sets and/or data modems 58, 60, 62 and 64. Other subscriber equipment, such as data processing devices or computers, is connected to the data modems.

The equipment at the subscriber premises 18 typically generates upstream signals 40 (FIG. 2) to the network interface device 10 for delivery to the CATV network 20. The upstream signals 40 may be either active or passive upstream signals generated by the subscriber equipment connected to the active and passive ports 45, 50, 52, 54 and 56. For example, one or more of the TV sets 58, 60, 62 and 64 may have conventional set top boxes (not shown) associated with them to allow the subscriber/viewer to make programming and viewing selections. Of course, any computers (not shown) connected to the data modems 58, 60, 62 and 64 typically communicate upstream signals. The telephone set 48 and the voice modem 46, or the EMTA (not shown), also generate upstream signals as a part of their typical functionality.

Electrical power for the network interface device 10 is supplied from a conventional DC power supply 66 connected to a dedicated power input port 68. Alternatively, electrical power can be supplied through a conventional power inserter (also shown at 58) that is connected to the port 50. The power inserter allows relatively low voltage DC power to be conducted through the same port 50 that also conducts high-frequency signals. Use of a conventional power inserter connected to one of the ports, e.g. port 50, eliminates the need for a separate dedicated power supply port 68, or provides an alternative port through which electrical power can also be applied. The power supply 66 or the power supplied from the port 50 is typically derived from a conventional wall outlet (not shown) within the subscriber premises 18.

The ports 44, 45, 50, 52, 54, 56 and 68 are each preferably formed by a conventional female coaxial cable connector which is mechanically connected to the housing 12 and which is electrically connected to internal components of the interface device 10. Coaxial cables from the subscriber equipment and the drop cables 38 (FIG. 2) are connected to the interface device 10 by mechanically connecting the corresponding mating male coaxial cable connector (not shown) on these coaxial cables to the female coaxial cable connectors forming the ports 44, 45, 50, 52, 54, 56 and 68.

The internal circuit components of one embodiment of the network interface device 10 are shown in FIG. 3. Those internal circuit components include a conventional bi-directional signal splitter/combiner 70 which separates the input downstream signals 22 from the CATV network 20 at the cable port 44 into passive downstream signals 72 and active downstream signals 74 within the network interface device 10. The passive downstream signals 72 are conducted directly through the passive port 45 to the passive subscriber equipment 46 and 48. Passive upstream signals 76 created by the passive subscriber equipment 46 and 48 are conducted through the passive port 45 directly to the signal splitter/combiner 70 to become upstream signals 40 in the CATV network 20. The direct signal conductivity path for the passive signals in the network interface device 10 avoids subjecting the passive signals to potentially adverse influences from electronic components that might fail or malfunction, thereby enhancing the reliability of the passive communications without increasing the risk of failure. Passive communications are intended to be as reliable as possible since they may be used in emergency and critical circumstances.

The active downstream signals 74 are conducted to active circuitry 78, where the active downstream signals 74 are amplified, filtered, modified, enhanced or otherwise conditioned before delivery through the active ports 50, 52, 54 and 56 to the subscriber equipment 58, 60, 62 and 64. Active upstream signals 80 are created by the subscriber equipment 58, 60, 62 and 64, and also pass through the active circuitry 78, where those signals are also conditioned or otherwise modified or enhanced before being combined at the signal splitter/combiner 70 to become network upstream signals 40 in the CATV network 20.

The circuit components of the active circuitry 78 receive power from the power supply 66 connected at port 68 or through the power inserter 58 (FIG. 1) connected at port 50. A conventional power-signal divider 82 separates the high-frequency active downstream and upstream signals 74 and 80 at port 50 from the DC power at port 50. The divider 82 conducts the active signals 74 and 80 from and to high-frequency signal conductivity paths within the active circuitry 78, while simultaneously conducting the DC power to the active circuitry 78 for use by its electrical power consuming components. Electrical power from the dedicated power input port 68 is also conducted to the power consuming circuit components of the active circuitry 78.

The components of the active circuitry 78 which conduct the downstream active signals 74 include first and second analog downstream filters 84 and 86 that are connected in series by a linear amplifier 88. The downstream filters 84 and 86 filter the downstream signals 74 in the downstream 54-1000 MHz frequency band. The linear amplifier 88 amplifies, modifies or enhances the downstream signals 74, and in conjunction with the filters 84 and 86, conditions the downstream signals 74. The downstream signals 74 are thereafter connected through conventional signal splitter/combiners 90, 92 and 94 before those downstream signals 74 are delivered through the active ports 50, 52, 54 and 56 to the subscriber equipment 58, 60, 62 and 64.

The active upstream signals 80 created by the subscriber equipment 58, 60, 62 and 64 are conducted through the active ports 50, 52, 54 and 56 to an upstream noise mitigating circuit 100. The upstream noise mitigation circuit 100 transfers valid active upstream signals 80 from the subscriber equipment 58, 60, 62 and 64 through the network interface device 10 to the CATV network 20 as upstream signals 40. These functions are accomplished as described below.

Valid upstream signals from the subscriber equipment 58, 60, 62 and 64 are conducted through the signal splitter/combiners 92, 94 and 90 to become active upstream signals 80. The upstream signals 80 are applied to a first upstream signal bandpass filter 102. Because the downstream signal filter 86 passes signals only in the 54-1000 MHz band, valid upstream signals 80 in the frequency band of 5-42 MHz are blocked by the downstream signal filter 86 and diverted through the upstream signal filter 102. The first upstream signal filter 102 preferably passes signals in the valid upstream signal frequency range of 5-42 MHz. Typical ingress noise falls within most intensely within the frequency range of 0-15 MHz, so the first upstream filter 102 has the capability of removing ingress noise at the low frequencies in the range of 0-5 MHz. However, ingress noise in the range of 5-15 MHz will be conducted by the upstream signal filter 102.

To mitigate or preventingress noise upstream signals from entering the CATV network 20 from the network interface device 10, ingress noise signals conducted through the first upstream filter 102 are isolated by a first radio frequency (RF) single pole double throw (SPDT) electronic switch 104 and terminated to ground through a termination resistor 103. The termination resistor 103 is connected to one terminal of the first electronic switch 104. Signals from the first upstream signal filter 102 are conducted through a conventional directional coupler 105 to and through the switch 104 to the termination resistor 103 while the first electronic switch 104 is in a normal position, shown in FIG. 3. All signals conducted through the first upstream signal filter 102 are terminated through the termination resistor 103, and are thereby prevented from entering the CATV network 20, while the first switch 104 is in its normal position.

The first electronic switch 104 changes to an alternate activated position (not shown in FIG. 3) upon the instantaneous power of the signals conducted through the filter 102 reaching a magnitude indicative of a valid upstream signal from the subscriber equipment 58, 60, 62 or 64. To distinguish relatively low power ingress noise from the relatively higher power of a valid upstream signal, the instantaneous magnitude of the power of the signals passing through the upstream filter 102 is detected and evaluated. The coupler 105 delivers a signal 106 which is typically 10 dB lower in power than the signal passing through the coupler 105 to the switch 104.

The signal 106 from the coupler 105 is conducted to an input terminal of a conventional log amplifier detector 108. The log amplifier detector 108 operates on an inverse logarithmic basis to convert the instantaneous magnitude of power of the signal 106 to a DC voltage output signal 110. By operating on an inverse logarithmic basis, the typical decibel power of the input signal 106 is converted into a linear DC voltage output signal 110 whose magnitude is inversely related to the instantaneous input power. This logarithmic conversion allows the log amplifier detector 108 to function as an instantaneous demodulating power detector whose output DC voltage signal is inversely proportional to the logarithm of the input power. A log amp detector 108 which is satisfactory for use in the present invention is part number AD 8319 available from Analog Devices of Norwood Mass., USA. The DC voltage output signal 110 therefore represents the inverse of the instantaneous power of the upstream signal 80 conducted through the directional coupler 105.

The DC voltage output signal 110 from the log amp detector 108 is applied to a negative input terminal of a comparator 112. A threshold signal 114 is applied to the positive input terminal of the comparator 112. The threshold signal 114 is derived from a resistor divider network such as a potentiometer 116 and a resistor 118 connected in series, or from another voltage source. Adjustment of the value of the potentiometer 116 adjusts the magnitude of the threshold signal 114. The adjustment of the threshold signal 114 establishes the level where an trigger signal 120 from the comparator 112 switches from a logic low level to a logic high level.

The magnitude of the DC voltage output signal 110 from the log amp detector 108 is inversely related to the magnitude of the instantaneous power of the upstream signal represented by signal 106. That is, when the magnitude of the upstream signal 106 is relatively large, the DC voltage output signal 110 from the log amp detector 108 is relatively small, and vice versa. Because of this inverse relationship, the DC voltage output signal 110 is applied to the negative input terminal of the comparator 112, and the threshold signal 114 is applied to the positive input terminal of the comparator 112. Applying the two input signals in this manner causes the comparator 112 to supply a logic high trigger signal 120 whenever the magnitude of the instantaneous power of the upstream signal exceeds a predetermined threshold power level representative of a valid upstream signal. Conversely, when the DC voltage output signal 110 is greater than the signal 114, the trigger signal 120 from the comparator 112 is at a logic low level. When the DC voltage output signal 110 is less than the signal 114, the trigger signal 120 from the comparator is at a logic high level. The logic high level of the signal 120 therefore represents the condition where the instantaneous power of the upstream signal exceeds the predetermined threshold power level established by the signal 114.

Upon sensing that the instantaneous power content of an upstream signal exceeds the level represented by the predetermined threshold power level, the upstream signal is immediately transmitted or passed to the CATV network 20 as a network upstream signal 40. Upstream signals which do not meet the threshold power level are considered ingress noise. Ingress noise signals are isolated from the CATV network 20 by the switches 104 and 130, while incident upstream signals 80 are simultaneously terminated to ground through the termination resistor 103. The functions of passing upstream signals to the CATV network and terminating upstream signals to ground are accomplished in response to the logic level of the trigger signal 120 from the comparator 112.

When instantaneous power content of an upstream signal exceeds the threshold power level, the resulting logic high signal 120 from the comparator 112 triggers a one-shot timer 122. Simultaneously, the logic high signal 120 is applied to an input terminal of an OR gate 124. The OR gate 124 responds by applying a logic high control signal 126 to the control terminals of the first SPDT RF electronic switch 104 and a second SPDT RF electronic switch 130. The electronic switches 104 and 130 normally occupy the positions shown in FIG. 3. Upon the assertion of logic high control signal 126, the switches 104 and 130 immediately change from their normal positions (shown in FIG. 3) to their opposite activated positions (not shown). The activated positions of the switches 104 and 130 establish a direct connection over conductor 132 between the switches 104 and 130. Since the electronic switches 104 and 130 switch with radio frequency speed, the switches 104 and 130 assume the activated position almost instantaneously in response to the assertion of the control signal 126.

The activated positions of the switches 104 and 130 conduct the upstream signal 80 from the first upstream signal filter 102 through the conductor 132 to a second upstream signal filter 134. Both filters 102 and 134 suppress frequencies other than those in the frequency band of 5-42 MHz. The valid upstream signal flows from the second upstream filter 134 through the signal splitter/combiner 70 into the cable network 20 as the network upstream signal 40. Terminating resistors 103 and 190 are connected to the filters 102 and 134 when the switches 104 and 130 are in their normal positions, and the filters 102 and 134 are connected together over the conductor 132 when the switches 104 and 130 are in their activated positions.

Valid upstream signals are conducted to the CATV network almost instantaneously when the instantaneous power level of the upstream signals exceeds the threshold power level. By responding almost instantaneously when the threshold power level is exceeded, the chances are minimized that the information contained in the valid upstream signal will be lost, as might be the case if the power of the upstream signal had to be integrated over a time period before a determination of a valid upstream signal could be made on the basis of energy content. Such integration raises the possibility that some of the information of the upstream signal will be lost and not transferred upstream. In contrast, no integration of the power of the upstream signal over a selected time period is required in the upstream noise mitigation circuit 100. By almost instantaneously transmitting upstream signals which have a power content that exceeds the predetermined threshold power level, the integrity of the information contained in the upstream signal is better preserved.

Once the switches 104 and 130 have been moved to the activated position which directly connects the first and second upstream signal filters 102 and 134 through the conductor 132, the switches 104 and 130 are maintained in this activated position for a time determined by the one-shot timer 122. When triggered by the logic high signal 120, the one-shot timer 122 immediately supplies a logic high output signal 136 to the OR gate 124. Either logic high signal 120 or 136 causes the OR gate 124 to supply the logic high control signal 126. If the power level of the upstream signal falls below the level of the threshold signal 114, the signal 120 immediately assumes a logic low level. However, the one-shot timer 122 will continue to deliver the logic high output signal 136 for the time duration of its internal time constant.

The internal time constant of the one-shot timer 122 is equal to the amount of time to transmit a single valid upstream signal packet of the maximum time duration permitted by the signaling protocol, plus a slight additional amount of time to account for inherent tolerances in the components and the timing of the one-shot timer 122. In this manner, the one-shot timer 122 ensures that the switches 104 and 130 assume their activated positions for a long enough time to conduct all single valid upstream signals, including a maximum-length valid upstream signal or packet.

The situation just described is illustrated by the waveform diagrams shown in FIG. 4, taken in connection with FIG. 3. The signal 106 represents a single valid upstream packet of the permitted maximum time duration whose detection by the log amp detector 108 produces the logic high trigger signal 120. The signal 120 assumes the logic high level at time point 138, triggering the one-shot timer 122 and causing the output signal 136 to be asserted at the same time point 138. The control signal 126 from the OR gate 124 immediately assumes a logic high level at time point 138. The electronic switches 104 and 130 assume their activated positions for the duration of the logic high control signal 126. At time point 139, the maximum time duration of a single valid upstream packet or signal ends, and the instantaneous power represented by that signal falls below the threshold power level represented by the threshold signal 114. The signal 120 assumes a logic low level. Since the time constant of one-shot timer 122 is established to slightly exceed the maximum time duration of a single valid upstream packet or signal, the logic high signal 136 will continue to time point 140. When the signal 136 assumes a logic low level after the one-shot timer 122 times out at time point 140, the control signal 126 from the OR gate 124 simultaneously assumes a logic low level. As a result, the control signal 126 is longer in duration than signal 120. When the control signal 126 assumes the low logic level at time point 140, the electronic switches 104 and 130 assume their normal positions to conduct any upstream signals to the termination resistor 103, thereby terminating those signals to ground and preventing the further upstream signals from reaching the CATV network.

For multiple valid upstream signal packets which are consecutively transmitted without a substantial time interval separating the multiple sequential upstream packets, the one-shot timer 122 will time out before the valid upstream signal transmission terminates. However, the continuous instantaneous power of the multiple sequential valid upstream signal packets will continue to exceed the threshold power level for the duration of the multiple sequential signal packets, thereby causing the comparator 112 to continue to assert the logic high trigger signal 120 to the OR gate 124 for the duration of the multiple sequential signal packets. The continued application of the logic high signal 120 causes the OR gate 124 to assert the logic high control signal 126 beyond the time when the one-shot timer 122 times out. The two upstream signal filters 102 and 134 remain connected by the switches 104 and 130 in their activated positions, and thereby conduct the multiple sequential upstream signal packets to assure that the full information represented by the multiple sequential signal packets is not truncated or lost by premature termination of those signals. At the termination of such multiple upstream signal packets, the signal power no longer exceeds the threshold signal 114, and the switches 104 and 130 immediately assume their normal positions, thereby preventing any ingress noise from entering the CATV network 20 after the longer or multiple sequential valid upstream packets have been transmitted.

The situation just described is illustrated by the waveform diagrams shown in FIG. 5, taken in conjunction with FIG. 3. The signal 106 represents three, for example, sequential valid upstream packets or signals. The trigger signal 120 assumes the logic high level at time point 142 in response to recognizing the first of the sequential valid upstream packets. The one-shot timer 122 is triggered and causes the output signal 136 to be asserted at time point 142. The control signal 126 from the OR gate 124 also assumes a logic high level at time point 142 in response to the assertion of the control signal 136. The electronic switches 104 and 130 assume their activated positions in response to the logic high control signal 126. At time point 140, the one-shot timer 122 times out, causing its output signal 136 to assume a logic low level. However, the instantaneous power level from the multiple sequential upstream signal packets continues to exceed the threshold power level, until the sequence of multiple upstream signal packets terminates at time point 146. So long as the signal 120 is at a logic high level, the control signal 126 from the OR gate 124 causes the electronic switches 104 and 130 to remain in the activated position, conducting the multiple sequential valid upstream signal packets to the CATV network 20. Once the sequence of multiple valid upstream signal packets has been transmitted, which occurs at time point 146, the absence of any further valid upstream signal causes the instantaneous power level to fall below the threshold power level, and the signals 120 and 126 assume a logic low level. The electronic switches 104 and 130 respond by assuming their normal positions to prevent the further transmission of upstream signals to the CATV network.

If the instantaneous power of ingress noise exceeds the threshold power level, the electronic switches 104 and 130 assume their activated positions, as can be understood from FIG. 3. An unusually high and short duration power level of ingress noise can cause this situation. Under that circumstance, the trigger signal 120 assumes a logic high level, and the one-shot timer 136 is triggered and asserts the output signal 136. The electronic switches 104 and 130 assume their activated positions, allowing the ingress noise to pass through the upstream filters 102 and 134. Until the one-shot timer 122 times out, ingress noise will be allowed to enter the CATV network 20. The effect of this ingress noise is minimized by the time constant of the one-shot timer 122 extending only for the maximum time duration of the longest single valid upstream signal packet permitted under the communication protocol.

The response to ingress noise having instantaneous power that exceeds the threshold is illustrated by the waveform diagrams shown in FIG. 6, taken in connection with FIG. 3. The ingress noise signal is shown at 106. Because the instantaneous power of the ingress noise exceeds the threshold, a logic high trigger signal 120 is asserted from the comparator 112 at time point 148, thereby triggering the one-shot timer 122 and causing the signal 136 to be asserted at the same time point 148. The logic high signal 136 causes the OR gate 124 to assert the logic high control signal 126 at time point 148. The electronic switches 104 and 130 assume their activated positions for the duration of the high level of the control signal 126. At time point 150, the instantaneous power from the ingress noise falls below the threshold power level, causing the comparator 112 to assert a logic low trigger signal 120. However, the one-shot timer 122 has not timed out and continues to deliver the logic high signal 136 for the time duration of its time constant, until time point 140. The control signal 126 from the OR gate 124 transitions to a logic low level at time point 140 when the one-shot timer 122 times out, causing the electronic switches 104 and 130 (FIG. 3) to assume their normal positions. The electronic switch 104 connects the termination resistor 103 to terminate any further upstream signals to ground and thereby prevent any further transfer of upstream signals to the CATV network.

An alternative form 160 of the upstream noise mitigation circuit, shown in FIG. 7, reduces the amount of time that ingress noise may be conducted to the CATV network 20 after the initial instantaneous power of the ingress noise is sufficient to exceed the threshold power level, compared to the response of the circuit 100 (FIG. 3). The upstream noise mitigation circuit 160 shown in FIG. 7 includes many of the same components as the upstream noise mitigation circuit 100 (FIG. 3), and those same components function in the manner previously described.

In response to the instantaneous power of the ingress noise exceeding the threshold power level, represented by signal 114, the comparator 112 supplies the logic high trigger signal 120, in the manner previously described. The logic high trigger signal 120 is applied to a one-shot timer 162, to the input terminal of a SPDT RF electronic switch 164, to a second one-shot timer 168, and to the set terminal of a set-reset latch 172. In response to the logic high signal 120, the first one-shot timer 162 triggers and supplies an output signal 166. Simultaneously, the second one-shot timer 168 is triggered and supplies a signal 170. The latch 172 is immediately set in response to the logic high trigger signal 120 and supplies the control signal 126 to the RF electronic switches 104 and 130, causing them to switch to their activated positions and establish the upstream signal communication path for conducting upstream signals through the upstream signal filters 102 and 134. In this manner, the noise mitigation circuit 160 responds almost instantaneously to the instantaneous power of the upstream signal exceeding the threshold to immediately conduct the upstream signal to the CATV network without delay and without the risk of diminishing or losing some of the information contained in the upstream signal. In this regard, the upstream noise mitigation circuit 160 (FIG. 7) is similar in initial response to the upstream noise mitigation circuit 100 (FIG. 3). However, the upstream noise mitigation circuit 160 has the capability of more quickly closing the upstream communication path through the switches 104 and 130 when the upstream communication path was initially established in response to ingress noise.

The rapid closure of the upstream communication path in response to ingress noise is accomplished by integrating the signal 120 for a predetermined time established by the time constant of the one-shot timer 162. The logic high trigger signal 120 represents the power of the ingress noise exceeding the predetermined threshold power level. Integrating the logic high trigger signal 120 results in a value which represents energy above the threshold power level for the time duration of integration. Integration occurs over the time that the signal 166 is asserted by the one-shot timer 162. If the amount of power integrated during this time, i.e. energy, is not sufficient to confirm a valid upstream signal with continuous sustained instantaneous power, the switches 104 and 130 are moved to their normal positions, thereby terminating the upstream communication path. Since ingress noise generally does not contain significant sustained energy even though an initial burst of the ingress noise may have sufficient instantaneous power to exceed the threshold, the upstream communication path is quickly closed in a typical ingress noise situation.

Integrating the power represented by the threshold power level is accomplished by an integration circuit 179. The integration circuit 179 includes an operational amplifier 176. The positive input terminal of the operational amplifier 176 is connected to ground reference. A capacitor 178 is connected between the negative input terminal and the output terminal of the operational amplifier 176. The negative input terminal of the operational amplifier 176 is the input point for signals to the integration circuit 179.

Prior to commencement of integration, the switch 164 is in its normal position shown in FIG. 7. In the normal position of the switch 164, a positive voltage signal 171 is conducted from a power supply source 175 to a resistor 174 which is connected to the negative input terminal of an operational amplifier 176. Applying the positive voltage to the negative input terminal of the operational amplifier 176 has the effect of causing integration across the capacitor 178 to establish an output signal 180 at a voltage level near the ground reference. A voltage level near the ground reference constitutes a logic low signal. Thus, in the normal position of the switch 164, the output signal 180 from the integrator circuit 179 is at a logic low level.

In response to the control signal 166 moving the switch 164 from its normal position shown in FIG. 7 to its activated position which is the alternate of that position shown in FIG. 7, the logic high trigger signal 120 is applied through the resistor 174 to the negative input terminal of the operational amplifier 176. So long as the trigger signal 120 is at the logic high level, the output signal 180 from the operational amplifier 176 remains at a logic low level. However, because ingress noise typically has the effect of rapidly subsiding in instantaneous power, the instantaneous power will usually not exceed the threshold for a significant sustained amount of time, thereby causing the signal 120 to assume a logic low level during the time that the one-shot timer 162 supplies the control signal 166. Consequently, with the switch 164 in the activated position and the signal 120 at a logic low level, the operational amplifier 176 integrates this change of input signal level across the capacitor 178, which causes the output signal 180 to start increasing from the ground reference level. If the instantaneous power of the ingress noise remains low for a significant portion of the time that the one-shot timer 162 asserts the control signal 166, as is typical with ingress noise having an initial momentarily-high instantaneous power burst, the voltage across the capacitor 178 will increase to a level which corresponds to a logic high level of the signal 180.

The logic high output signal 180 is applied to one input terminal of an AND gate 167. The control signal 166 is applied to another input terminal of the AND gate 167. The input terminal to which the control signal 166 is applied is an inverting input terminal, thereby causing the AND gate 167 to respond to the inverted logic level of the control signal 166. The signal 180 remains at a logic high level for a time period after integration ceases from the integration circuit 179, and the control signal 166 assumes the logic low level at the end of the integration time established by the one-shot timer 162. At that point, the AND gate 167 responds to two logic high signals (the logic low signal 166 is inverted at the input terminal), resulting in a logic high level signal 169 applied to an OR gate 182. The OR gate 182 supplies a logic high level signal 184 to a reset terminal of the latch 176. The latch 176 resets, and de-asserts the control signal 126 to the switches 104 and 130, thereby closing the upstream communication path through the upstream filters 102 and 134. Thus, soon after the initial instantaneous power of the ingress signal diminishes and the integration time set by the one-shot timer 162 expires, the upstream communication path is closed to the further conduction of upstream signals, thereby preventing any further ingress noise from entering the CATV network.

During the time and situation just described, another AND gate 185 has no effect on the functionality. The signal 170 supplied by the one-shot timer 168 is asserted for a considerably longer period of time than the one-shot timer 162 asserts the control signal 166. The time of assertion of the signal 170 is the length of time, plus a margin for component tolerances, of the longest single valid upstream packet or signal permitted under the signal communication protocol. The time of integration represented by the assertion of the control signal 166 is considerably less than the longest single valid upstream packet. During the integration of the instantaneous power of the ingress noise over the time duration of the control signal 166, the output signal 170 is at a logic high level, the control signal 126 is at a logic high level because the latch 172 will have been set by the trigger signal 120, before the signal 120 assumes a logic low level after the initial high instantaneous power of the ingress noise has dissipated. The input terminals of the AND gate 185 to which the signals 120 and 170 are applied are inverting. Thus, under these conditions, the AND gate 185 supplies an output signal 187 at a logic low level.

The situation of terminating the upstream communication path created by a burst of ingress noise before expiration of the time duration of a maximum-length valid upstream signal or packet is illustrated by the waveform diagrams shown in FIG. 8, taken in connection with FIG. 7. The ingress noise signal is shown at 106. The instantaneous power of the ingress noise exceeds the threshold power level and causes a logic high trigger signal 120 from the comparator 112 at time point 148, thereby triggering the one-shot timers 162 and 168 and causing the control signals 166 and 170 to be asserted at the time point 148. The control signal 126 from the latch 172 also assumes a logic high level at time point 148 because the logic high trigger signal 120 sets the latch 172. The electronic switches 104 and 130 assume their activated positions for the duration of the logic high control signal 126 to maintain the upstream communication path. At time point 150, the instantaneous power of the ingress noise falls below the threshold power level, and the trigger signal 120 assumes a logic low level. However, the first one-shot timer 162 has not timed out and continues to deliver the control signal 166 until it times out at time point 188. The time duration between time points 148 and 188 is the time constant of the one-shot timer 162 which establishes the time duration of integration. The time for integrating a valid upstream signal is the time between time points 148 and 188.

If the integrated value indicates an upstream signal of unsustained instantaneous power, consistent with ingress noise that rapidly dissipates, the resulting logic high signal 180 from the integrator 179 is applied to the OR gate 182. The OR gate 182 supplies the logic high signal 180 at time point 188 which, when logically anded with the logical inversion of signal 166, causes the AND gate 167 to assert the signal 169. The OR gate 182 responds by asserting a logic high signal 184, which resets the latch 172, thereby de-asserting the control signal 126. The upstream communication path is terminated when the switches 104 and 130 assume their normal positions.

As is understood from FIG. 8, the upstream communication path remains open from time point 148 to time point 188. This time is considerably less than the maximum time length of a single valid upstream packet or signal, represented by the time between points 148 and 189, or between time points 148 and 150 (FIG. 6). Consequently, even though the upstream communication path is immediately established to allow upstream signal communication whenever the instantaneous power exceeds the threshold, that upstream communication path is closed to further upstream communication very rapidly thereafter if spurious ingress noise established that communication path.

Whenever an upstream signal has sustained instantaneous power, the noise mitigation circuit 160 assures that the upstream signal will be conducted to the CATV network. Such circumstances indicate a valid upstream signal. As understood from FIG. 7, the trigger signal 120 is asserted at a logic high level when the valid upstream signal exceeds the threshold. The latch 172 is set and asserts the logic high control signal 126 which moves the switches 104 and 132 their activated positions to establish the upstream communication path. The timers 162 and 168 are triggered, and the one-shot timer 162 moves the switch 164 to its activated position. The output signal 180 remains at a logic low level during the time of a valid upstream signal while the one-shot timer 162 asserts the control signal 166 and while the logic high trigger signal 120 remains at a logic high level due to the sustained instantaneous power of the valid upstream signal exceeding the threshold. The logic low signal 180 and the inversion of the logic high signal 166 at the input terminal of the AND gate 167 causes the AND gate 167 to assert a logic low signal 169, which has no effect on the OR gate 182 or the latch 172. Thus, during the transmission of a valid upstream signal, the AND gate 167 has no effect on the status of the latch 172.

On the other hand, the time constant of the one-shot timer 168 is considerably longer than the time constant of the one-shot timer 162. The signal 170 from the timer 168 is asserted for the time duration of a single valid maximum-length upstream packet or signal. The logic high level of the signal 170 is inverted at the input terminal of the AND gate 185. At this time, the control signal 126 is at a logic high level because the latch 172 has been set. The continuous instantaneous power of the valid upstream signal is represented by a logic high level of the trigger signal 120. The logic high level of the signal 120 is inverted at the AND gate 185. The logic level of the signals applied to the AND gate 185 causes it to supply a logic low signal 187, which has no effect on the latch 172 during conditions of sustained instantaneous power from the valid upstream signal.

When the valid upstream signal terminates, the logic high level of the signal 120 changes to a logic low level. The logic low level signal 120 is inverted at its input terminal to the AND gate 185. The logic high signal 170 is still asserted by the one-shot timer 168, because the timer 168 times the duration of a single valid maximum-length upstream signal. Until the one-shot timer 168 de-asserts the signal 170, the AND gate 185 will not assert a logic high signal 187. However, when the signal 170 is de-asserted, the AND gate 185 applies the logic high signal 187 to the OR gate 182. The OR gate 182 asserts the signal 184 to reset the latch 172, and the control signal 126 is de-asserted. The switches 104 and 132 move to their normal positions and terminate the upstream communication path through the filters 102 and 134.

In response to sustained instantaneous power representative of a valid upstream signal, the noise mitigation circuit 160 assures that an upstream communication path will be established for the maximum time duration of a single valid upstream signal, provided that there is sufficient instantaneous energy in the upstream signal during the integration time established by the signal 166. In this manner, the circuit 160 is similar to the circuit 100 (FIG. 3) which assures that the upstream communication path remains established for the time duration of a single valid maximum-length upstream signal or packet. However, unlike the circuit 100 (FIG. 3) the circuit 160 discriminates between short-duration high instantaneous power ingress noise and continuous-duration high instantaneous power upstream signals and rapidly terminates the upstream communication path in response to the former.

The situation of maintaining the upstream communication path in response to sustained instantaneous energy of an upstream signal during the integration time established by the time constant of the one-shot timer 162, to allow adequate time for a single valid upstream packet of maximum duration to be transmitted, is illustrated by the waveform diagrams shown in FIG. 9, taken in connection with FIG. 7. The upstream signal is represented by a packet having a time duration less than the maximum allowed time duration for single valid upstream packet as shown at 106. The instantaneous power of the upstream packet 106 exceeds the threshold power level and causes a logic high trigger signal 120 from the comparator 112 at time point 148, thereby triggering the one-shot timers 162 and 168 and causing the control signals 166 and 170 to be asserted at the same time point 148. The control signal 126 from the latch 172 also assumes a logic high level at time point 148 due to the assertion of the logic high signal 120. The electronic switches 104 and 130 assume their activated positions for the duration of the logic high signal 126 and establish the upstream communication path. At time point 188, the first one-shot timer 162 times out and de-asserts the control signal 166. The time duration between time points 148 and 188 establishes the time duration of integration.

During the time of integration, the instantaneous power of the single packet 106 continuously exceeds the threshold level. Consequently, the output signal 180 from the integration circuit 179 remains at a logic low level, and the inversion of the control signal 166 at the AND gate 167 maintains the output signal 169 in a logic low level. At time point 188 when the one-shot timer 162 times out, the control signal 166 assumes a logic low level, but the inversion of that logic low level at the input terminal to the AND gate 167, coupled with the continuous logic low level signal 180 continues to maintain the output signal 169 at a logic low level. The logic low signal 169 does not change for the duration of the situation shown in FIG. 9. As a result, the AND gate 167 has no effect on resetting the latch 172 in this situation.

During the time between points 148 and 188, the logic high control signal 126, the logic high trigger signal 120, which is inverted at its input terminal to the AND gate 185, and the logic high control signal 170, which is also inverted at its input terminal to the AND gate 185, cause the output signal 187 from the AND gate 185 to remain at a logic low level. Therefore, during this time between points 148 and 188, the signal 187 from the AND gate 185 has no effect on resetting the latch 172. At time point 190 the packet 106 terminates. The instantaneous power associated with the packet 106 also terminates, causing the trigger signal 120 to achieve a logic low level. However, the one-shot timer 168 has not yet timed out, so its output signal 170 remains at a logic high level until time point 189. The logic low level trigger signal 120 does not change the state of the AND gate 185. Consequently, the latch with 172 remains set at time point 190.

When the one-shot timer 168 times out, at point 189, the control signal 170 assumes a low logic level. The low logic signal 170 is inverted at its input terminal to the AND gate 185. The trigger signal 120 previously assumed a logic low level at time point 190. The inversion of the signals 120 and 170 at the input terminals to the AND gate 185 results in three logic high input signals to the AND gate 185, causing the output signal 187 to assume a logic high level. The logic high signal 187 is applied to the OR gate 182, and the output signal 184 from the OR gate resets the latch 172.

Upon reset, the latch 172 de-asserts the control signal 126 at time point 189, thereby closing the upstream communication path through the filters 102 and 134 as a result of the switches 104 and 130 assuming their normal positions. Thus, as is understood from FIG. 9, a valid upstream signal of any duration will exceed the minimum power threshold measured during the integration time established by the one-shot timer 162, and as a consequence, the latch 172 will continue to assert the control signal 126 and maintain the upstream communication path through the filters 102 and 104. The upstream communication path will be maintained for the duration of the time constant of the one-shot timer 168, during which its output signal 170 is asserted at a logic high level. By maintaining the upstream communication path during the time that the one-shot timer 168 asserts the control signal 170, it is assured that all valid upstream signals having a time length at least equal to the maximum length of a single valid upstream signal will pass through the upstream communication path. Consequently, none of the information contained in a single valid upstream packet will be lost or truncated.

The upstream signal communication path remains established during the time between the actual end of the valid upstream packet and the end of a maximum-length valid upstream packet, represented by the difference in time between points 190 and 189, but that amount of time is relatively short and maintenance of the upstream communication path during this time assures that a valid upstream signal packet of any length up to the maximum length will be transmitted without loss or truncation of any of its information.

In addition to the previously described advantages of quickly closing the upstream communication path after it was established by ingress noise and of establishing the upstream communication path for the maximum length of a valid upstream signal, the noise mitigation circuit 160 also has the capability of transmitting multiple sequential valid data packets, without loss or truncation of information. This situation can be understood by reference to FIG. 10, taken in conjunction with FIG. 7.

The first valid upstream packet of the multiple sequence of valid upstream packets, shown at 106 in FIG. 10, establishes the upstream communication path due to its sustained instantaneous energy. This energy is sustained during the integration time established by the one-shot timer 162. The control signal 166 is asserted at a high logic level until time point 188, and the control signal 170 is asserted at a high logic level until time point 189.

The instantaneous power of the sequence of multiple valid upstream packets remains above the threshold level and the trigger signal 120 remains asserted at a logic high level for the duration of that sequence of packets until time point 193, when the instantaneous power of the multiple sequential upstream packets terminates. The one-shot timer 168 does not time out until time point 189, at which point its output signal 170 assumes a logic low level at time point 189. The low logic level of the control signal 170 is inverted at its input terminal to the AND gate 185. However, at time point 189, the states of the input signals to the AND gate 185 result in the AND gate 185 supplying a logic low output signal 187. The logic low output signal 187 has no effect on the OR gate 182 and the latch 172 remains set.

At time point 193, the instantaneous power of the sequence of multiple valid upstream packets 106 falls below the threshold, causing the trigger signal 120 to assume a logic low level. The logic low level of the signal 120 at time point 193 is inverted at its input terminal to the AND gate 185, causing the AND gate to assert a logic high output signal 187. The logic high signal 187 causes the OR gate 182 to assert the signal 184, thereby resetting the latch 172 and de-asserting the signal 126. The switches 104 and 130 assume their normal positions, thereby terminating the communication path through the upstream signal filters 102 and 134.

In this manner, the upstream communication path is maintained for the duration of the multiple sequential packets, represented by the time between points 148 and 193. However, after the last packet in the multiple sequential series of valid upstream packets ends, the upstream communication path is closed to the further transmission of upstream signals, thereby preventing ingress noise from entering the CATV network.

As has been described in conjunction with FIGS. 7-10, any upstream signal, whether a valid upstream signal or ingress noise, which has sufficient instantaneous power to exceed the threshold will immediately open the upstream communication path through the filters 102 and 134. In this sense, the noise mitigation circuit 160 does not distinguish between a valid upstream signals and invalid ingress noise which may have sufficient energy to exceed the threshold. Not distinguishing between these signals assures that there is no delay in transmitting valid upstream signals. A delay in transmitting valid upstream signals could lose or truncate part of the information contained in those valid signals. However, once the upstream communication path has been established, the sustained instantaneous power of the upstream signal is integrated during the integration time established by the one-shot timer 162, between time points 148 and 188. If the instantaneous power of the upstream signal is not sustained, as is the typical case with ingress noise, the upstream communication path is terminated thereafter at time point 188. On the other hand, if the instantaneous power of the upstream signal is sustained during the integration time, as is the typical case with a valid upstream signal of any duration, the upstream communication path is maintained for the maximum duration of a single valid upstream signal or packet, represented by the time between points 148 and 189. In this manner, an upstream communication path is assured for the time duration necessary to transmit a single valid upstream packet of maximum time duration established by the communication protocol. Again, no loss or truncation of information of any valid upstream packet is assured. Similarly, there is no loss or truncation of the information contained in a sequence of multiple valid upstream packets, even when the multiple sequential upstream packets have a time duration which exceeds the maximum time duration of a single valid upstream packet. The upstream communication path remains open for the duration of the multiple sequential upstream packets, represented by the time between points 148 and 193. However as soon as the instantaneous power represented by the multiple upstream sequential packets falls below the threshold, at time point 193, the upstream communication path is terminated to prevent any ingress noise from entering the CATV network at the conclusion of the multiple sequential upstream packets.

The benefit of the termination resistors 103 and 190 is their ability to avoid signal reflections, as understood from FIGS. 3 and 7. The proclivity for high-frequency signals to reflect is related to the impedance characteristic of the termination of the conductor which conducts those signals and to the frequency of those signals, as is well known. For this reason, coaxial cables are typically terminated by connecting a terminating impedance between the signal-carrying center conductor and the surrounding reference plane shielding. The terminating impedance value should have a value equal to a characteristic impedance between the signal-carrying conductor and the reference plane shielding, to minimize signal reflections.

The values of the termination resistors 103 and 190 are selected to equal the characteristic impedance of the coaxial cables which form the drop cables 38 (FIG. 2), and that value is typically 75 ohms. Matching the value of the termination resistors 103 and 190 to the characteristic impedance of the coaxial cables minimizes the amount of signal reflection. Reflected signals combine with the incident downstream signals and cancel or degrade the downstream signals. Minimizing the signal reflection maximizes the quality and fidelity of the downstream signals and enhances the quality of service provided from the CATV network.

A further significant feature is the incorporation of a gas tube surge protection device 192 in the network interface device 10, as shown in FIG. 3. The gas tube surge protection device 192 is an integral component and is permanently enclosed within the housing 12 (FIG. 1). The gas tube surge protection device 192 provides protection against destruction of and damage to the components of the interface device 10 which typically might arise from lightning strikes to the CATV network 20 or from other unanticipated high voltage and high current applications to the CATV network. Because the infrastructure of the CATV network extends over a considerable geographical area, a lightning strike or other unexpected high voltage, high current application may adversely affect or destroy electronic components in the CATV network infrastructure, including the interface devices 10. For this reason, industry standards require some form of surge protection.

The typical previous types of surge protectors are inductor-capacitor circuits, metal oxide varistors, and avalanche diodes. These devices may be made a part of a network interface device, or these devices are included in cable taps 36 (FIG. 2). Inductor-capacitor circuits, metal oxide varistors and avalanche diodes only offer effective protection against relatively lower voltage and lower current surges. Inductor-capacitor circuits, metal oxide varistors and avalanche diodes are susceptible to failure in response to higher voltage and higher current surges, such as those arising from lightning strikes. Of course, the failure of such devices eliminates any protection and usually leads to failure of the components within the CATV network and within the network interface device. The CATV service provider is required to replace failed network interface devices, but a failed surge protector may not be recognized until after the destruction of other components has occurred.

Grounding blocks are another previous form of surge protection. Grounding blocks are devices used in cable taps 36 (FIG. 2), and include conductors which provide a common ground reference among the various devices within the cable taps 36. Grounding blocks may also be used in connection with a gas tube surge protection device within the cable taps 36, but gas tube surge protection devices are not commonly used with grounding blocks because of the relative expense associated with such devices and the perceived satisfactory protection available from the common grounding connection. The other disadvantage of using a gas tube surge protection device with a grounding block is that the arrangement is not fully effective. The gas tube surge protection device is located at the cable taps 36 (FIG. 2), but the cable taps 36 are separated by drop cables 38 from the network interface devices 10. A lightning strike or other surge condition unexpectedly applied to one of the drop cables 38 will be conducted directly to the interface device 10 which has no surge protection, as well as to the cable tap 36. Any protection provided by the grounding block, whether or not it includes a gas tube surge protection device, is not assuredly available to the network interface device 10, because the adverse surge can be conducted directly to the network interface device 10 and avoid the gas tube surge protection device in the cable tap 36.

Incorporating the gas tube surge protection device 192 in the network interface device 10, as shown in FIG. 3, offers a greater capability to protect against higher voltage and higher current surges and against repeated surges. The gas tube surge protection device 192 remains functional in response to higher voltage and higher current surges than can be responded to by inductor-capacitor circuits, metal oxide varistors and avalanche diodes. The gas tube surge protection device 192 also offers a capability to resist a greater number of multiple surges compared to other known previous devices. While the previous devices may respond to a moderate number of moderate level surges, the number of such responses is limited. After that number is exceeded, such previous devices tend to fail even in response to moderate surge conditions.

Locating the gas tube surge protection device 192 in the network interface device 10 provides the best level of protection against high voltage and high current surges arising within the CATV network infrastructure and arising from active and passive subscriber equipment connected to the network interface device 10. Downstream surges will be suppressed as they enter the network interface device 10 from the CATV network infrastructure. Even though it is unlikely that a surge condition will originate at the subscriber equipment connected to the interface device 10, the gas tube surge protection device 192 will provide protection for the other components within the CATV network 20 from upstream surges.

Incorporating the gas tube surge protection device 192 in the network interface device 10 also offers economic advantages, which are translated into a lower cost to the CATV service provider. The increased cost arising from incorporating the gas tube surge protection device 192 in the network interface device 10 is more than offset by avoiding the necessity to occasionally replace entire failed network interface devices and/or other components within the CATV network infrastructure. A gas tube surge protection device which is satisfactory for use in the network interface device is part number BAS230V supplied by CITEL INC, of Miami, Fla., USA.

As described above, there are numerous advantages and improvements available from the present invention. The upstream noise mitigation circuits (100 and 160, FIGS. 3 and 7) respond to the instantaneous power of upstream signals. When the instantaneous power exceeds a predetermined threshold, a signal path for conducting the upstream signal to the CATV network is immediately established. Establishing the upstream communication path immediately when the instantaneous power of the upstream signal exceeds the threshold substantially reduces or diminishes the risk that information contained in the upstream signal will be lost, truncated or diminished. The risk of truncating or losing information in the upstream signal is considerably reduced or diminished compared to devices which integrate the power of the upstream signal over a time period before establishing the upstream communication path. By responding to the instantaneous power, the information in valid upstream signals is preserved. On the other hand, the upstream noise mitigation circuits 100 and 160 (FIGS. 3 and 7) offer the capability of quickly isolating and terminating the upstream communication path and thereby minimizing the ingress noise entering the CATV network.

In addition, the incorporation of the gas tube surge protection device within the network interface device itself offers substantial protective and economic advantages over the previous known uses of surge protection devices for CATV networks.

Many other advantages and improvements will be apparent upon gaining a complete appreciation for the present invention. The preferred embodiments of the invention and many of its improvements have been described with a degree of particularity. This detailed description is of preferred examples of implementing the invention and is not necessarily intended to limit the scope of the invention. The scope of the invention is defined by the following claims. 

1. A network interface device for connecting subscriber equipment to a cable television (CATV) network over which downstream signals in a first frequency band from a headend of the CATV network are transmitted to the subscriber equipment and valid upstream signals in a second different frequency band are transmitted from the subscriber equipment to the headend, the network interface device having an upstream noise mitigation circuit which mitigates ingress noise into the CATV network in the second frequency band, comprising: an upstream filter which filters upstream signals before delivery to the CATV network; a detector connected to the upstream filter which determines an instantaneous level of power of the upstream signals filtered by the upstream filter, and which supplies a power signal related to the instantaneous power of the upstream signals; a threshold circuit which establishes a threshold level of power which characterizes valid upstream signals, and which supplies a threshold signal related to the established threshold level of power; a comparator connected to the detector and the threshold circuit which compares the power signal to the threshold signal, and which asserts a trigger signal when the power signal exceeds the threshold signal; a switch connected to the upstream filter and having a normal position and an activated position, the normal position isolating and terminating signals from the upstream filter and the activated position conducting the upstream signals from the upstream filter to the CATV network, the switch assuming the activated position in response to the assertion of a control signal and assuming the normal position in response to the de-assertion of the control signal; and a control circuit connected to the comparator and to the switch, the control circuit asserts a control signal to the switch immediately upon assertion of the trigger signal and de-asserts the control signal to the switch at or after de-assertion of the trigger signal.
 2. A network interface device as defined in claim 1, wherein the control circuit further comprises: a timer receptive of the trigger signal and operative to assert a timing signal for a predetermined time period commencing upon the assertion of the trigger signal; and wherein: the control circuit responds to the timing signal to de-assert the control signal when the trigger signal is de-asserted or upon expiration of the predetermined time period.
 3. A network interface device as defined in claim 2, wherein: the predetermined time period established by the timer is approximately as long as an amount of time required to transmit a valid maximum-length upstream signal.
 4. A network interface device as defined in claim 3, wherein: the control circuit de-asserts the control signal after the trigger signal is de-asserted and upon expiration of the predetermined time period.
 5. A network interface device as defined in claim 4, wherein the control circuit further comprises: an integrator circuit which integrates the trigger signal over a predetermined integration time to achieve an integration value; and wherein: the control circuit responds to the integration value to de-assert the control signal when the integration value is less than a preestablished value.
 6. A network interface device as defined in claim 5, wherein: the predetermined integration time is less than the predetermined time period.
 7. A network interface device as defined in claim 6, wherein: the control circuit de-asserts the control signal in response to a first one of the following conditions to occur: (a) the integration value is less than the preestablished value at the end of the predetermined integration time, (b) at the expiration of the predetermined time period if the trigger signal has been de-asserted at or before the expiration of the predetermined time period, and (c) at the de-assertion of the trigger signal.
 8. A network interface device as defined in claim 7, wherein the control circuit further comprises: a first timer which establishes the predetermined integration time; a second timer which establishes the predetermined time period; and a latch which sets to assert the control signal in response to the assertion of the trigger signal and which resets in response to a reset signal; a gate which logically combines signals representative of the conditions (a) and (b) and (c) and asserts the reset signal to the latch in response to the first one of the conditions to occur.
 9. A network interface device as defined in claim 1, wherein: the upstream filter constitutes a first upstream filter; the switch constitutes a first switch; the first upstream filter is connected to the first switch; and the noise mitigation circuit further comprises: a second upstream filter which filters upstream signals before delivery to the CATV network; a second switch connected to the first switch and to the second upstream filter, the second switch having a normal position and an activated position, the normal position of the second switch terminating the second upstream filter, and the activated position of the second switch conducting the upstream signals from the first switch and the first upstream filter through the second upstream filter to the CATV network; and wherein: the first and second switches assume activated positions in response to the assertion of the control signal and assume normal positions in response to the de-assertion of the control signal.
 10. A network interface device as defined in claim 9, further comprising: a signal splitter/combiner connected to the first and second upstream filters which divides downstream signals from the CATV network into active and passive downstream signals within the network interface device and which combines passive upstream signals from passive subscriber equipment and active upstream signals from active subscriber equipment into upstream signals transmitted to the CATV network; and wherein: the passive downstream signals are transmitted through the network interface device without substantial conditioning to the passive subscriber equipment and the passive upstream signals are transmitted through the network interface device without substantial conditioning to the CATV network; the active downstream signals are conditioned when transmitted through the network interface device to the active subscriber equipment and the active upstream signals are conditioned when transmitted through the network interface device to the CATV network; the active upstream signals are conducted through the first and second upstream filters to the signal splitter/combiner for transmission to the CATV network when the first and second switches are in their activated positions; and ingress noise in the second frequency band originating from active subscriber equipment is conducted through the first upstream filter and is isolated and terminated when the first switch is in the normal position.
 11. A network interface device as defined in claim 9, wherein a drop cable connects the network interface device to the CATV network and the drop cable has a characteristic impedance value, and the upstream noise mitigation circuit further comprises: a first termination resistor connected to terminate the first upstream filter when the first switch is in the normal position; a second termination resistor connected terminate to the second upstream filter when the second switch is in the normal position; and wherein: the first and second termination resistors have impedance values substantially equal to a characteristic impedance of the drop cable.
 12. A network interface device as defined in claim 1, further comprising: a gas tube surge protection device connected within the network interface device where the upstream and downstream signals are conducted to and from the CATV network, the gas tube surge protection device operative to shunt high voltage and high current surges from the components and subscriber equipment.
 13. A network interface device comprising components which conduct upstream and downstream signals to and from a cable television (CATV) network and subscriber equipment, the network interface device further comprising: a gas tube surge protection device connected within the network interface device where the upstream and downstream signals are conducted to and from the CATV network, the gas tube surge protection device operative to shunt high voltage and high current surges from the components and subscriber equipment.
 14. A method of mitigating upstream ingress noise to a cable television (CATV) network, comprising: transmitting downstream signals in a first frequency band from a headend of the CATV network to subscriber equipment; transmitting valid upstream signals in a second different frequency band from the subscriber equipment to the headend; filtering the upstream signals including ingress noise to confine the frequency of the upstream signals to the second frequency band; determining an instantaneous power content of the upstream signals; establishing a threshold power level which characterizes valid upstream signals from the subscriber equipment; comparing the instantaneous power content to a threshold power level; conducting the filtered upstream signals to the CATV network whenever the instantaneous power content is equal to or greater than the threshold power level; and preventing the filtered upstream signals from reaching the CATV network when the instantaneous power content is less than the threshold power level.
 15. A method as defined in claim 14, further comprising: conducting the filtered upstream signals to the CATV network for a predetermined time period after the instantaneous power content equals or exceeds the threshold power level.
 16. A method as defined in claim 15, further comprising: establishing the predetermined time period as approximately equal to an amount of time required to transmit a valid maximum-length upstream signal.
 17. A method as defined in claim 16, further comprising: integrating a value representative of the instantaneous power content over a predetermined integration time to arrive at a integration value; and preventing the filtered upstream signals from reaching the CATV network after expiration of the predetermined integration time if the integration value is less than a predetermined threshold power level.
 18. A method as defined in claim 17, further comprising: establishing the predetermined integration time as less than the length of time required to transmit a single valid minimum-length upstream signal.
 19. A method as defined in claim 18, further comprising: blocking the filtered upstream signals from reaching the CATV network after expiration of a predetermined amount of time after initiation of the predetermined amount of integration time if the integration value is less than a predetermined threshold power level; and the predetermined amount of time is substantially equal to the amount of time required to transmit a single valid maximum-length upstream signal.
 20. A method as defined in claim 14, further comprising: dividing downstream signals from the CATV network into active downstream signals and passive downstream signals at premises of a subscriber; transmitting the passive downstream signals to passive subscriber equipment; transmitting the active downstream signals to active subscriber equipment; conditioning the active upstream signals from the active subscriber equipment; not conditioning the passive upstream signals from the passive subscriber equipment; combining non-conditioned passive upstream signals and the conditioned active upstream signals into upstream signals transmitted to the CATV network; determining an instantaneous power content of the active upstream signals; and preventing noise in the second frequency band originating from active subscriber equipment from reaching the CATV network when the instantaneous power content of the active upstream signals is less than the threshold power level. 